It’s taken three years, dozens of phone calls and hundreds — maybe a thousand — pieces of documents, but Grant Harper Reid can now say he knows all there is to know about his granddad.

Reid’s grandfather was showman Leonard Harper, a hoofer/producer who was instrumental in introducing the likes of Duke Ellington, the Mills Brothers and Louis Armstrong to the world.

Nearly 100 years ago, Harper was such a showbiz giant that he helped integrate vaudeville, Reid said, becoming, in 1921, the first black to play the Shubert circuit.

“All I knew of him is, my mother told me he had a show in England once,” said Reid, whose accumulated reams of vintage program covers, handouts, letters and photos of Harper are tucked away in loose-leaf binders.

“I lived my whole life not knowing about him.”

Reid said Harper started performing as a 12-year-old in a girl’s dress, not uncommon in 1913, and went on to produce some 2,000 shows between 1920 and 1943, when he died at age 44.

“He was the inventor of the nightclub form of entertainment,” said Reid, 46, a Harlem free-lance movie locator. “He’d have eight shows going at different clubs at the same time — shows at white clubs, shows uptown, plus a tap show on tour.”

Harper roomed with Ellington and brought him to the Cotton Club, Reid said, and had a young Louis Armstrong in his “Hot Chocolates” revue — in which Satchmo’s solo “became the biggest thing in the show.”

From talking to old-timers and visiting archives and libraries, Reid also discovered that Harper assisted black film pioneer Oscar Micheaux make his first talkie (“The Exile” in 1931), produced the first Cotton Club revues, and staged shows at Al Capone’s Chicago version of the Cotton Club, the Grand Terrace Cafe.

Unfortunately, by the 1930s, white audiences weren’t taking the A train uptown anymore, and Ellington and Count Basie’s bands had made the variety shows obsolete overnight. Harper never did adjust.

“When his form of entertainment died, so did he,” Reid noted.

Today, Reid said he wants Harper’s legacy to come to life on the 27-inch screen, if not the larger, silver version.

“Nobody has told his story,” Reid said. “And I’d like to do a ‘Roots’ type of thing for TV.”